Who has experience here with the use of a Pontoon boat in Colorado Rivers?

With rating of Class I (easy rivers for beginners) to Class VI (extreme river drifting) I would like to know what kind of Pontoon Boat I should get?

I see the Colorado River mentioned, the Gunnison, the Arkansas, and of course there are many others. Which Rivers do you feel confident in using a Pontoon Boat in, and what Pontoon in Specific, can you trust?

When the rating refers to a Class IV river, does it mean you just get a Bouncy Ride, or does it mean you have about a 50% to 50% chance of getting overturned in the river?